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Abstract:

A method of forming a graphene oxide based layer includes preparing a
dispersion of graphene oxide and nanostructures, and spin coating the
dispersion on a surface of a substrate to form a spin coated film
thereon; and thermally annealing the spin coated film to form the
graphene oxide based layer, where the mass ratio of the graphene oxide
and the nanostructures in the graphene oxide based layer is in a range of
about 1:0.01 w/w to 1:0.8 w/w. The nanostructures are functionalized with
carboxylic acid. The nanostructures include carbon nanotubes, or
nanofibers. The carbon nanotubes include single walled carbon nanotubes
(SWCNTs) or multi-walled carbon nanotubes (MWCNTs).

Claims:

1. A method of forming a graphene oxide (GO) based layer, comprising the
steps of: (a) preparing a solution of GO; (b) preparing a solution of
nanostructures; and (c) applying the solution of GO and the solution of
nanostructures onto a surface of a substrate to form a GO based layer
with the plurality of nanostructures.

2. The method of claim 1, wherein the substrate is an indium tin oxide
(ITO) layer.

5. The method of claim 3, wherein the nanostructures are functionalized
with carboxylic acid.

6. The method of claim 1, wherein the step of preparing the solution of
GO is performed by dispersing the GO in deionized water.

7. The method of claim 1, wherein the step of preparing the solution of
nanostructures comprises the steps of: (a) dispersing the nanostructures
in water to form a mixture thereof; (b) ultrasonicating the mixture for a
first period of time; and (c) centrifugating the ultrasonicated mixture
at a predetermined speed for a second period of time.

8. The method of claim 7, wherein the first period of time is about 2
hours, the second period of time is about 1 hours, and the predetermined
speed is about 11,000 rpm.

9. The method of claim 1, wherein the step of applying the solution of GO
and the solution of nanostructures onto the surface of the substrate
comprises the steps of: (a) mixing the solution of GO and the solution of
nanostructures to form a dispersion of the GO and the nanostructures; (b)
spin coating the GO and nanostructures dispersion on the surface of the
substrate to form a spin coated film of the GO and nanostructures
dispersion thereon; and (c) thermally annealing the spin coated film to
form the GO based layer.

10. The method of claim 1, wherein a mass ratio of the GO and the
nanostructures in the GO based layer is in a range of about 1:0.01 w/w to
1:0.8 w/w.

11. A method of forming a graphene oxide (GO) based layer, comprising the
steps of: (a) preparing a dispersion of GO and nanostructures; and (b)
spin coating the dispersion on a surface of a substrate to form a spin
coated film thereon; and (c) thermally annealing the spin coated film to
form the GO based layer.

12. The method of claim 11, wherein the substrate is an indium tin oxide
(ITO) layer.

13. The method of claim 11, wherein the step of preparing the dispersion
of GO and nanostructures comprises the steps of: (a) mixing the GO and
the nanostructures in water to form a mixture thereof; and (b)
ultrasonicating the mixture to form the dispersion of GO and nano
structures.

[0003] Some references, which may include patents, patent applications and
various publications, are cited and discussed in the description of this
invention. The citation and/or discussion of such references is provided
merely to clarify the description of the present invention and is not an
admission that any such reference is "prior art" to the invention
described herein. All references cited and discussed in this
specification are incorporated herein by reference in their entireties
and to the same extent as if each reference was individually incorporated
by reference. In terms of notation, hereinafter, "[n]" represents the nth
reference cited in the reference list. For example, [7] represents the
7th reference cited in the reference list, namely, L. J. Cote, F. Kim, J.
X. Huang, J. Am. Chem. Soc. 2009, 131, 1043.

FIELD OF THE INVENTION

[0004] The invention generally relates to materials for composite and
electronics applications, and more specifically, to water processable
graphene oxide:single walled carbon nanotube composite, and relevant
applications of the same, such as an anode modifier for polymer solar
cells.

BACKGROUND OF THE INVENTION

[0005] The development of affordable photovoltaic technologies, i.e.,
solar cells, offers one of the promising solutions for clean, renewable
energy. Solution-processed polymer photovoltaic cells have attracted much
attention since they can be manufactured with inexpensive,
high-throughput techniques [1]. They are light weight and can also be
flexible, making it possible to integrate them with ordinary surfaces at
a large scale to provide inexpensive power sources. They can also be
customized to serve as cheap, disposable, soft generators for remote,
autonomous micro devices. With a rapid progress in both materials design
and device architectures, it is envisioned that polymer photovoltaic
system with power conversion efficiency (PCE) over 10% can be realized
[2].

[0006] In a typical polymer solar cell, the transparent anode, e.g.,
indium tin oxide (ITO), is usually coated with a modifying layer that can
block electrons but help to transport holes to minimize carrier
recombination. Among the successful hole transporting layer materials,
the conjugated polymer
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has
been predominantly used as an anode modifier due to its exceptional
solution processability [1]. Li et al. first reported that graphene oxide
(GO) can also be used as a hole transporting layer on ITO for polymer
solar cells, potentially replacing PEDOT:PSS [3]. GO is a graphene
derivative decorated with oxygen-containing groups such as carboxylic
acids, epoxides and hydroxides that can be readily synthesized by
oxidative exfoliation of graphite powders to yield stable aqueous
dispersion of single layer sheets [4-6]. The apparent thickness of a GO
sheet was measured to be around 1 nm by atomic force microcopy (AFM) but
its lateral dimension can readily extend to tens of microns [7, 8]. The
large band gap of GO helps block electron flows towards the anode, thus
effectively suppressing the carrier recombination [3, 9]. The performance
of the GO hole transporting layer was demonstrated to be comparable to
PEDOT:PSS. However, since GO is insulating, it also increases the
internal resistance of the device, which could lower the fill factor
unless its thickness can be kept at the minimum, ideally down to one
monolayer. An earlier work of the inventors has shown that GO pieces can
be tiled up by Langmuir-Blodgett assembly to obtain high coverage
monolayers [7, 10]. Since spin coating is routinely used for fabricating
polymer solar cells, it would be a more desirable method if very thin GO
modifying layers can be made. Li et al. used spin coating to make the GO
modifying layers on ITO surface and found that only when the thickness of
the GO films was reduced to around 2 nm, were their performances
comparable to PEDOT:PSS [3]. However, spin coating such thin GO films
(i.e., one to two monolayers) can be extremely challenging because the
resulting films also need to fully coverage the underlying surface.

[0007] Therefore, a heretofore unaddressed need exists in the art to
address the aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

[0008] In one aspect, the invention relates to a method of forming a GO
based layer. In one embodiment, the method includes the steps of
preparing a solution of GO; preparing a solution of nanostructures; and
applying the solution of GO and the solution of nanostructures onto a
surface of a substrate to form a GO based layer with the plurality of
nanostructures. The mass ratio of the GO and the nanostructures in the GO
based layer is in a range of about 1:0.01 w/w to 1:0.8 w/w.

[0009] In one embodiment, the substrate is an ITO layer.

[0010] The nanostructures include carbon nanotubes, or nanofibers, where
the nanostructures are functionalized with carboxylic acid. In one
embodiment, the carbon nanotubes include single walled carbon nanotubes
(SWCNTs) or multi-walled carbon nanotubes (MWCNTs).

[0011] In one embodiment, the step of preparing the solution of GO is
performed by dispersing the GO in deionized water.

[0012] In one embodiment, the step of preparing the solution of
nanostructures comprises the steps of dispersing the nanostructures in
water to form a mixture thereof; ultrasonicating the mixture for a first
period of time; and centrifugating the ultrasonicated mixture at a
predetermined speed for a second period of time, where the first period
of time is about 2 hours, the second period of time is about 1 hours, and
the predetermined speed is about 11,000 rpm.

[0013] In one embodiment, the step of applying the solution of GO and the
solution of nanostructures onto the surface of the substrate comprises
the steps of mixing the solution of GO and the solution of nanostructures
to form a dispersion of the GO and the nanostructures; spin coating the
GO and nanostructures dispersion on the surface of the substrate to form
a spin coated film of the GO and nanostructures dispersion thereon; and
thermally annealing the spin coated film to form the GO based layer.

[0014] In another aspect, the invention relates to a method of forming a
GO based layer. In one embodiment, the method includes the steps of
preparing a dispersion of GO and nanostructures, and spin coating the
dispersion on a surface of a substrate to form a spin coated film
thereon; and thermally annealing the spin coated film to form the GO
based layer. The mass ratio of the GO and the nanostructures in the GO
based layer is in a range of about 1:0.01 w/w to 1:0.8 w/w.

[0015] In one embodiment, the nanostructures include carbon nanotubes, or
nanofibers, where the nanostructures are functionalized with carboxylic
acid. In one embodiment, the carbon nanotubes include SWCNTs or MWCNTs.

[0016] In one embodiment, the step of preparing the dispersion of GO and
nanostructures comprises the steps of mixing the GO and the
nanostructures in water to form a mixture thereof; and ultrasonicating
the mixture to form the dispersion of GO and nanostructures.

[0017] In one embodiment, the substrate is an ITO layer.

[0018] In yet another aspect, the invention relates to an article of
manufacture including a substrate having a surface; and a composite layer
of GO and nanostructures formed on the surface of the substrate. In one
embodiment, the nanostructures include carbon nanotubes, or nanofibers,
where the nanostructures are functionalized with carboxylic acid. In one
embodiment, the carbon nanotubes include SWCNTs or MWCNTs. In one
embodiment, the mass ratio of the GO and the nanostructures in the GO
based layer is in a range of about 1:0.01 w/w to 1:0.8 w/w.

[0019] In one embodiment, the article of manufacture is a solar cell,
wherein the substrate is an ITO layer. The ITO layer is pre-patterned,
such that the pre-patterned ITO layer and the composite layer form an
anode. In one embodiment, the solar cell further comprises an active
layer formed on the GO and nanostructure composite layer; and a cathode
formed on active layer.

[0020] In a further aspect, the invention relates to an article of
manufacture including a composite of GO and nanostructures, where a mass
ratio of the GO and the nanostructures in the composite is in a range of
about 1:0.01 w/w to 1:0.8 w/w.

[0021] In one embodiment, the nanostructures include carbon nanotubes, or
nanofibers, where the nanostructures are functionalized with carboxylic
acid. In one embodiment, the carbon nanotubes include SWCNTs or MWCNTs.

[0022] These and other aspects of the present invention will become
apparent from the following description of the preferred embodiment taken
in conjunction with the following drawings, although variations and
modifications therein may be affected without departing from the spirit
and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings illustrate one or more embodiments of the
invention and together with the written description, serve to explain the
principles of the invention.

[0024] Wherever possible, the same reference numbers are used throughout
the drawings to refer to the same or like elements of an embodiment.

[0025]FIG. 1 shows (a) current density-voltage characteristics of
P3HT:PCBM solar cells with GO as the hole transporting layer (inset). A
small change in the processing condition of GO can significantly and
indeed, unexpectedly, affect the performance of the devices. (b)-(d) AFM
images of GO thin films deposited on SiO2/Si substrates. As the
concentration of the starting dispersion increases from (b) about 0.1 wt
% to (c) about 0.15 wt %, the surface coverage of the resulting films
greatly improves, leading to more reproducible devices (a, open circle).
(d) about 0.2 wt % dispersion can produce thin films with near full
coverage, but the increase in GO thickness from (c) about 1 nm to (d)
about 3-4 nm resulted in substantial decrease in device performance (a,
open triangle).

[0026] FIG. 2 shows addition of a small amount of SWCNTs into GO
(GO:SWCNTs=1:0.05, w/w) can increase, even prominently, the FF and
JSC in devices fabricated with GO modifiers prepared from (a) about
0.15 wt % and (b) about 0.2 wt % dispersion. Note that the improvement is
more pronounced with the thicker GO film (b). (c) AFM image of the
GO:SWCNTs layer, showing very low surface roughness (root mean square
roughness=0.962 nm). (d) The vertical resistance of the GO film was
decreased by an order of magnitude after adding SWCNTs, leading to
decreased serial resistance in the final device and increased FF.

[0029]FIG. 5 shows schematically a cross-sectional view of a photovoltaic
device according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown. This invention may, however, be
embodied in many different forms and should not be construed as limited
to the embodiments set forth herein. Rather, these embodiments are
provided so that this disclosure will be thorough and complete, and will
fully convey the scope of the invention to those skilled in the art. Like
reference numerals refer to like elements throughout.

[0031] The terms used in this specification generally have their ordinary
meanings in the art, within the context of the invention, and in the
specific context where each term is used. Certain terms that are used to
describe the invention are discussed below, or elsewhere in the
specification, to provide additional guidance to the practitioner
regarding the description of the invention. For convenience, certain
terms may be highlighted, for example using italics and/or quotation
marks. The use of highlighting has no influence on the scope and meaning
of a term; the scope and meaning of a term is the same, in the same
context, whether or not it is highlighted. It will be appreciated that
same thing can be said in more than one way. Consequently, alternative
language and synonyms may be used for any one or more of the terms
discussed herein, nor is any special significance to be placed upon
whether or not a term is elaborated or discussed herein. Synonyms for
certain terms are provided. A recital of one or more synonyms does not
exclude the use of other synonyms. The use of examples anywhere in this
specification including examples of any terms discussed herein is
illustrative only, and in no way limits the scope and meaning of the
invention or of any exemplified term. Likewise, the invention is not
limited to various embodiments given in this specification.

[0032] It will be understood that, as used in the description herein and
throughout the claims that follow, the meaning of "a", "an", and "the"
includes plural reference unless the context clearly dictates otherwise.
Also, it will be understood that when an element is referred to as being
"on" another element, it can be directly on the other element or
intervening elements may be present therebetween. In contrast, when an
element is referred to as being "directly on" another element, there are
no intervening elements present. As used herein, the term "and/or"
includes any and all combinations of one or more of the associated listed
items.

[0033] It will be understood that, although the terms first, second, third
etc. may be used herein to describe various elements, components,
regions, layers and/or sections, these elements, components, regions,
layers and/or sections should not be limited by these terms. These terms
are only used to distinguish one element, component, region, layer or
section from another element, component, region, layer or section. Thus,
a first element, component, region, layer or section discussed below
could be termed a second element, component, region, layer or section
without departing from the teachings of the present invention.

[0034] Furthermore, relative terms, such as "lower" or "bottom" and
"upper" or "top," may be used herein to describe one element's
relationship to another element as illustrated in the Figures. It will be
understood that relative terms are intended to encompass different
orientations of the device in addition to the orientation depicted in the
Figures. For example, if the device in one of the figures is turned over,
elements described as being on the "lower" side of other elements would
then be oriented on "upper" sides of the other elements. The exemplary
term "lower", can therefore, encompasses both an orientation of "lower"
and "upper," depending of the particular orientation of the figure.
Similarly, if the device in one of the figures is turned over, elements
described as "below" or "beneath" other elements would then be oriented
"above" the other elements. The exemplary terms "below" or "beneath" can,
therefore, encompass both an orientation of above and below.

[0035] It will be further understood that the terms "comprises" and/or
"comprising," or "includes" and/or "including" or "has" and/or "having"
when used in this specification, specify the presence of stated features,
regions, integers, steps, operations, elements, and/or components, but do
not preclude the presence or addition of one or more other features,
regions, integers, steps, operations, elements, components, and/or groups
thereof.

[0036] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this invention
belongs. It will be further understood that terms, such as those defined
in commonly used dictionaries, should be interpreted as having a meaning
that is consistent with their meaning in the context of the relevant art
and the present disclosure, and will not be interpreted in an idealized
or overly formal sense unless expressly so defined herein.

[0037] As used herein, "around", "about" or "approximately" shall
generally mean within 20 percent, preferably within 10 percent, and more
preferably within 5 percent of a given value or range. Numerical
quantities given herein are approximate, meaning that the term "around",
"about" or "approximately" can be inferred if not expressly stated.

[0038] As used herein, the terms "comprising," "including," "carrying,"
"having," "containing," "involving," and the like are to be understood to
be open-ended, i.e., to mean including but not limited to.

[0039] As used herein, a "nanostructure" refers to an object of
intermediate size between molecular and microscopic (micrometer-sized)
structures. In describing nanostructures, the sizes of the nanostructures
refer to the number of dimensions on the nanoscale. For example,
nanotextured surfaces have one dimension on the nanoscale, i.e., only the
thickness of the surface of an object is between 0.1 and 1000 nm.
Nanotubes have two dimensions on the nanoscale, i.e., the diameter of the
tube is between 0.1 and 1000 nm; its length could be much greater.
Finally, sphere-like nanoparticles have three dimensions on the
nanoscale, i.e., the particle is between 0.1 and 1000 nm in each spatial
dimension. A list of nanostructures includes, but not limited to,
nanoparticle, nanocomposite, quantum dot, nanofilm, nanoshell, nanofiber,
nanoring, nanorod, nanotube, and so on.

[0040] As used herein, if any, the term "atomic force microscopy" or its
abbreviation "AFM" refers to a very high-resolution type of scanning
probe microscopy, with demonstrated resolution on the order of fractions
of a nanometer, more than 1000 times better than the optical diffraction
limit.

[0041] As used herein, if any, the term "scanning electron microscope" or
its abbreviation "SEM" refers to a type of electron microscope that
images the sample surface by scanning it with a high-energy beam of
electrons in a raster scan pattern. The electrons interact with the atoms
that make up the sample producing signals that contain information about
the sample's surface topography, composition and other properties such as
electrical conductivity.

[0042] As used herein, if any, the term "X-ray photoelectron spectroscopy"
or its abbreviation "XPS" refers to a method used to determine the
composition of the top few nanometers of a surface. It involves
irradiating a material with a beam of X-rays while simultaneously
measuring the kinetic energy and number of electrons that escape from the
top 1 to 10 nm of the material being analyzed.

[0043] The description will be made as to the embodiments of the present
invention in conjunction with the accompanying drawings in FIGS. 1-5. In
accordance with the purposes of this disclosure, as embodied and broadly
described herein, this disclosure, in one aspect, relates to an article
of manufacture having a composite of GO and nanostructures such as
SWCNTs, methods of making and applications of the same.

[0044] The nanostructures include carbon nanotubes, or nanofibers. The
carbon nanotubes include SWCNTs or MWCNTs. In one embodiment, the
nanostructures are functionalized with carboxylic acid, or the likes.
According to the invention, a mass ratio of the GO and the nanostructures
in the GO composite is in a range of about 1:0.01 w/w to 1:0.8 w/w.

[0045] In one embodiment, the article of manufacture is a solar cell, as
shown in FIG. 5, where the composite of GO and nanostructures is spin
coated on a surface of a substrate 110 to form a GO and nanostructure
composite layer 120 thereon. The substrate 110 is an ITO layer. The ITO
layer can be pre-patterned (not shown). The ITO layer 110 and the
composite layer 120 form an anode of the solar cell 100. The solar cell
100 also has an active layer 130 formed on the GO and nanostructure
composite layer 120, and a cathode 140 formed on active layer 130.

[0046] According to the invention, adding a small amount of
nanostructures, such as single walled carbon nanotubes (SWCNTs), in a GO
modifying layer/film can significantly improve the devices' fill factor
(FF). As discussed below, polymer solar cells fabricated with an
optimized GO:SWCNTs composite layer/film show comparable or even a bit
higher performance compared to the devices using PEDOT:PSS as the hole
transporting layer. Allowing thicker GO films to be used as the hole
transporting layer can greatly facilitate the integration of GO into
polymer photovoltaic devices.

[0047] In another aspect of the invention, a method of forming a GO based
layer includes preparing a solution of GO; preparing a solution of
nanostructures; and applying the solution of GO and the solution of
nanostructures onto a surface of a substrate to form a GO based layer
with the plurality of nanostructures. The mass ratio of the GO and the
nanostructures in the GO based layer is in a range of about 1:0.01 w/w to
1:0.8 w/w.

[0048] The nanostructures include carbon nanotubes, or nanofibers, where
the nanostructures are functionalized with carboxylic acid. In one
embodiment, the carbon nanotubes include SWCNTs or MWCNTs.

[0049] In one embodiment, the step of preparing the solution of GO is
performed by dispersing the GO in deionized water.

[0050] In one embodiment, the step of preparing the solution of
nanostructures comprises the steps of dispersing the nanostructures in
water to form a mixture thereof; ultrasonicating the mixture for a first
period of time; and centrifugating the ultrasonicated mixture at a
predetermined speed for a second period of time, where the first period
of time is about 2 hours, the second period of time is about 1 hours, and
the predetermined speed is about 11,000 rpm.

[0051] In one embodiment, the step of applying the solution of GO and the
solution of nanostructures onto the surface of the substrate comprises
the steps of mixing the solution of GO and the solution of nanostructures
to form a dispersion of the GO and the nanostructures; spin coating the
GO and nanostructures dispersion on the surface of the substrate to form
a spin coated film of the GO and nanostructures dispersion thereon; and
thermally annealing the spin coated film to form the GO based layer.

[0052] In a further aspect of the invention, a method of forming a GO
based layer includes the steps of preparing a dispersion of GO and
nanostructures, and spin coating the dispersion on a surface of a
substrate to form a spin coated film thereon; and thermally annealing the
spin coated film to form the GO based layer. The mass ratio of the GO and
the nanostructures in the GO based layer is in a range of about 1:0.01
w/w to 1:0.8 w/w.

[0053] In one embodiment, the step of preparing the dispersion of GO and
nanostructures comprises the steps of mixing the GO and the
nanostructures in water to form a mixture thereof; and ultrasonicating
the mixture to form the dispersion of GO and nanostructures.

[0054] In one embodiment, the nanostructures include carbon nanotubes, or
nanofibers, where the nanostructures are functionalized with carboxylic
acid. In one embodiment, the carbon nanotubes include SWCNTs or MWCNTs.

[0055] Without intent to limit the scope of the invention, further
exemplary processes and their related results according to the
embodiments of the present invention are given below.

[0056] Materials:

[0057] GO was synthesized by a modified Hummers' method [23] and purified
by a two-step washing method as reported elsewhere [22]. The purified GO,
with typical lateral size of a few microns, was dried and redispersed in
deionized water to create dispersions. SWCNTs were purchased from Carbon
Solutions, Inc., (P3-SWNT, functionalized with about 1.0-3.0 atomic %
carboxylic acid) and dispersed in water (1 mg/ml) by ultrasonication
(Misonix, S-4000) for about 2 hours, followed by centrifugation at about
11,000 rpm for about 1 hour (Eppendorf, 5417C) to collect the supernatant
for later use. PEDOT:PSS (Clevios 4083, H. C. Starck),
1,2-dichlorobenzene (anhydrous, Sigma-Aldrich), P3HT (4002-EE, Rieke
Metals, Inc.), and PCBM (>99.5%, Sigma-Aldrich) were used as received.

[0058] Device Fabrication:

[0059] GO:SWCNTs dispersions were spin coated on pre-patterned ITO anodes
(about 20 Ω/sq) at about 2,000 rpm for about 40 seconds, followed
by thermal annealing at about 120° C. for about 20 minutes. For
control devices, PEDOT:PSS was spin coated at about 4,000 rpm for about 1
minute, followed by thermal annealing at about 150° C. for about
30 minutes. The modified substrates were then transferred into a
nitrogen-filled glove box, and about 2 wt % P3HT:PCBM (1:1, w/w) in
1,2-dichlorobenzene was spin cast at about 600 rpm for about 40 seconds.
The active layers were then solvent-annealed by a slow-growth method for
about 30 minutes [24]. Finally, Ca/Al (about 20/60 nm) electrodes were
thermally evaporated to complete the devices.

[0060] Characterizations:

[0061] Two-terminal measurements were performed by using a Keithley 2400
source meter. Photovoltaic measurements were done by illuminating devices
(device area=about 0.04 cm2) using an Oriel Xe solar simulator (AM
1.5G simulated spectrum) equipped with an Oriel 130 monochromator.
Filters were used to cut off grating overtones. A calibrated silicon
reference solar cell with a KG5 filter certified by the National
Renewable Energy Laboratory was used to confirm the measurement
conditions. At least 25 devices were fabricated for each condition. To
minimize the uncertainty with the fabrication processes, the best 15
devices were used to calculate the average values of photovoltaic
parameters. The current density-voltage plot of a device that shows
closest performance to the average was chosen to represent the group in
the figures. Modifying layers deposited on the ITO were imaged by SEM
(Hitachi FE-SEM 54800) to examine their overall coverage and uniformity.
Due to the higher roughness of the ITO surface (c.a. 10 nm), the
microstructure of the GO and GO:SWCNTs thin films were examined by AFM
(Veeco, Multimode V) on samples deposited on smoother SiO2/Si
substrates. Under the same processing conditions, the GO thin films
deposited on SiO2/Si and ITO showed very similar overall coverage
and thickness. The transmittance was measured with a UV/vis spectrometer
(Agilent Technologies, 8653).

[0062] First, the effects of solution processing conditions for spin
coating GO thin films on the device performance were investigated, where
the prototypical poly(3-hexylthiophene):[6,6]-phenyl C61 butyric
acid methyl ester (P3HT:PCBM) bulk heterojunction devices [2] was chosen
as the model system since they have been extensively studied and
developed to a relatively more mature stage. In a device fabrication
process, an aqueous GO stock dispersion was diluted to various
concentrations and spin coated on pre-patterned ITO substrates at about
2,000 rpm. Then a P3HT:PCBM active layer was deposited on the top of the
modified ITO substrate, followed by solvent annealing and vacuum
deposition of Ca and Al to complete the device, as shown in inset of FIG.
1(a). Although the relationship between the GO layer thickness and the
device performance was already discussed in Li's paper [3], the
processing conditions of the GO modifying layer, e.g., dispersion
concentration and spin-coating parameters, were not provided. These
processing conditions are crucial to the device performance. A small
variation in the concentrations of the GO dispersion from about 0.1 wt %
to about 0.2 wt % can result in large variations in thin film coverage
and thickness, thus greatly affecting the device performance. Below about
0.15 wt %, the GO dispersion yielded non-uniform, incomplete coverage of
GO pieces, as shown in an AFM image in FIG. 1(b). Such films were found
to produce poor and irreproducible device performances. With about 0.15
wt % GO dispersion, it started to yield nearly complete coverage of GO,
as shown in FIG. 1(c), leading to reproducible photovoltaic results, as
shown in FIG. 1(a) (open circle). The open-circuit voltage (VOC),
short-circuit current density (JSC), FF, and PCE of the devices were
measured to be about 0.60 V, about 9.3±0.73 mA cm-2, about
60.1±3.5%, and about 3.28±0.14%, respectively. A slight increase of
the GO concentration to about 0.2 wt % yielded thicker GO thin films
(c.a. 3-4 nm) including stacked multilayers, as shown in FIG. 1(d). The
corresponding JSC, FF, and PCE of the devices decreased to about
9.05±0.67 mA cm-2, about 43.9±4.8%, and about 2.36±0.28%,
respectively, while the VOC remained the same, as shown in FIG. 1(a)
(open triangle). The decrease in JSC and FF is attributed to
increased vertical resistance with the thicker insulating GO film. This
shows that the processing conditions of GO have to be limited to a very
narrow window to ensure both near complete coverage and near monolayer
thickness for achieving a high device performance. This can be very
challenging due to the sheet-like morphology of GO, and its tendency to
wrinkle and overlap under capillary forces [10, 11].

[0063] Since it is much easier to achieve a complete coverage of GO with
larger thickness, if thicker films can be made more conductive along the
thickness direction, they could work more reliably as the modifying
layer. Although GO can be deoxygenated to greatly increase its
conductivity, the reduction process also alters its work function, making
it unsuitable as a hole transporting layer [12, 13]. Incorporating a more
conductive material such as SWCNTs in GO could solve the problem. The
small diameter of SWCNTs (c.a. 1 nm) makes them particularly attractive
as an adduct for GO thin films because it would not significantly
increase the surface roughness if they can be well dispersed. An earlier
work by the inventors discovered that GO can function as a surfactant to
disperse unfunctionalized SWCNTs in water [14, 15]. Meanwhile, the SWCNTs
also help to create more uniform GO thin films by stitching GO sheets
through π-π stacking [16]. The synergistic co-assembly between GO
and unfunctionalized SWCNTs can be explored to create hybrid thin films.
According to the invention disclosed below, the functionalized SWCNTs are
utilized. The water dispersible, surface functionalized SWCNTs are
sufficiently conductive to improve GO's performance as an anode modifier.
Such SWCNTs are functionalized with about 1.0-3.0 atomic % of carboxylic
acid groups, and can be readily mixed with GO to yield a uniform
GO:SWCNTs composite thin film by spin coating, which is favorable for
material processing in fabricating polymer solar cells.

[0064] The GO:SWCNTs dispersions were created by direct mixing of the two
components in water by sonication. Surprisingly, incorporating a small
amount of SWCNTs (GO:SWCNTs=1:0.05, w/w) into the GO modifying layer, as
shown in inset of FIG. 2(a), resulted in, unexpectedly, considerable
improvement for devices made with both thin GO modifying layers with
about 0.15 wt % GO, as shown in FIG. 2(a), (JSC=9.79±0.43 mA
cm-2, FF=62.7±2.1%, PCE=3.66±0.18%) and thick GO modifying
layers with about 0.2 wt % GO, as shown in FIG. 2(b),
(JSC=9.19±0.48 mA cm2, FF=56.4±2.3%, PCE=3.13±0.11%).
An AFM observation of the GO:SWCNTs thin films, as shown in FIG. 2(c),
shows that the overall thickness is largely unchanged after the
incorporation of SWCNTs. The GO:SWCNTs thin films also have very low
surface roughness (root mean square roughness=0.962 nm for FIG. 2(c)),
which is crucial for depositing smooth sequential layers to avoid
shorting. The AFM image also shows that the amount of the incorporated
SWCNTs (GO:SCNWTs=1:0.05, w/w) was below the percolation threshold.
Indeed, it was found that adding this amount of SWCNTs does not improve
the lateral conductivity GO thin films. However, the presence of the
SWCNTs was found to greatly improve the vertical conductivity of the GO
layer, which is more relevant to the charge flow in the thin film
photovoltaic devices. The inset of FIG. 2(d) shows the geometry of the
current-voltage measurement of the GO:SWCNTs thin film sandwiched between
Al and ITO electrodes. As shown in FIG. 2(d), the vertical resistance of
the GO layer (open circle) was reduced by an order of magnitude after
adding SWCNTs (solid circle). This leads to lower overall serial
resistance of the entire cell, thus improving FF and JSC. In
addition, adding SWCNTs greatly reduces the sensitivity of the device
performance on the thickness of the GO modifying layer. For example,
without SWCNTs, the PCEs of the cells decreased from about 3.28±0.14%
to about 2.36±0.28% when the thickness of GO increased from around
about 1 nm (spin coated from about 0.15 wt % GO dispersion) to around
about 3-4 nm (spin coated from about 0.2 wt % GO dispersion),
respectively. However, after adding SWCNTs, the difference between the
PCEs of the two types of cells (about 3.66±0.18% and about
3.13±0.11%, respectively) became much smaller. Overall, adding SWCNTs
leads to improved and more reproducible performances of GO modifying
layers that are also less sensitive to the solution processing
parameters.

[0065] According to the invention, with water processability of both GO
and the functionalized SWCNTs, it is convenient to tune their relative
fraction in the dispersion and thus in the final thin films, too. Table 1
below summarizes the effects of SWCNT concentration in the dispersion on
the device performances.

[0066] The GO concentration was kept at about 0.15 wt % to achieve an
optimized PCE. The improvement in FF appeared to be already saturated
after adding about 1:0.05 of SWCNTs into the GO layer. However, with
thicker GO layers, continued improvements in FF are achieved with higher
SWCNTs contents. As the SWCNT concentration was increased from about
1:0.05 to about 1:0.2, the VOC of the cells remained unchanged,
while JSC continued to increase, leading to improved PCE values. The
initial improvement of JSC is attributed to reduced vertical
resistance by adding SWCNTs, however, further improvement of JSC is
attributed to improved hole extraction from the polymer since the SWCNTs
added to the near monolayer thick GO thin film were likely in contact
with the active layer as well [17]. With about 1:0.2 GO:SWCNTs mass
ratio, the device PCE reached about 4.10±0.18%, which represents an
about 25% improvement over devices without SWCNTs. However, when the
SWCNT concentration was further increased to about 1:0.5 mass ratio, all
photovoltaic parameters, including VOC, dropped to lower values.
FIG. 3 shows the morphological evolution of the GO:SWCNTs thin films with
increased nanotube content revealed by AFM and scanning electron
microscopy (SEM). SEM images (FIGS. 3(a) and 3(b), left and upper right
panels) show that adding SWCNTs at about 1:0.2 mass ratio does not affect
the overall coverage and uniformity of the GO layer. The SWCNTs can be
seen to be evenly dispersed throughout the GO layer (FIG. 3(b), AFM,
lower right panel). However, with excessive amount of SWCNTs (about 1:0.5
mass ratio), the GO:SWCNTs films become non-uniform with islands of
multilayer GO:SWCNTs stacks as well as uncovered areas, as shown in FIG.
3(c). The AFM image reveals that at this concentration, the SWCNTs form
network-like aggregates, trapping thicker piles of GO pieces. Uneven
coverage and thickness of modifying layers at high SWCNT content are
responsible for their lower performance.

[0067]FIG. 4(a) shows the current density-voltage characteristics of
solar cells made with ITO modified by GO:SWCNTs (about 1:0.2 mass ratio,
spin coated from about 0.15 wt % GO dispersion), GO only (spin coated
from about 0.15 wt % GO dispersion), PEDOT:PSS, and an unmodified ITO
anode. In general, as shown in FIG. 4, the PEDOT:PSS devices (open
diamond) consistently show higher JSC and FF values than the devices
with only GO as modifier (open circle) and those without modifying layers
(open inversed triangle). However, the GO:SWCNTs devices (solid circle)
have even a bit higher JSC and comparable FF, leading to comparable
or even slightly higher PCE values. A potential advantage of using GO
based anode modifying layer is its high transparency in the red to near
infrared region of the solar spectrum, as shown in FIG. 4(b), which
allows more absorption by the active layer. The absorption of the active
layer P3HT:PCBM is insignificant in the red to near infrared, which makes
it an unsuitable system to fully explore this advantage. If GO based
modifying layers are shown to be compatible with low band gap polymer
systems [18], their performances even surpass PEDOT:PSS.

[0068] In summary, according to the invention, the GO:SWCNTs thin films
are found to be efficient anode modifying layers for polymer solar cells
using P3HT:PCBM as a model system. Adding a non-percolating amount of
SWCNTs into GO significantly improves its vertical conductivity, which
allows the use of thicker and thus easier-to-make GO films. This helps
reduce the strong dependence of the device performance on GO thickness
and solution processing conditions, leading to improved and more
reproducible results. The GO:SWCNTs thin films according to the invention
offer comparable performances to the conventional PEDOT:PSS anode
modifying layer. In addition, incorporating p-type SWCNTs into GO may
further promote hole extraction and the charge flow in the modifying
layer [19]. Since uniform GO:SWCNTs thin films with near complete
coverage can be made with much smaller thickness (around a few nm) than
PEDOT:PSS (typically 40 nm), and offer considerably higher optical
transmission in the longer wavelengths, the GO:SWCNTs thin films are an
attractive candidate as anode modifier for low band gap polymer solar
cells and possibly the interconnect layer [20, 21] in tandem devices.
Since GO can undergo partial deoxygenation upon gentle heating [22],
further work may be needed to evaluate the long term stability of devices
with GO based modifying layers.

[0069] The foregoing description of the exemplary embodiments of the
invention has been presented only for the purposes of illustration and
description and is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Many modifications and
variations are possible in light of the above teaching.

[0070] The embodiments were chosen and described in order to explain the
principles of the invention and their practical application so as to
enable others skilled in the art to utilize the invention and various
embodiments and with various modifications as are suited to the
particular use contemplated. Alternative embodiments will become apparent
to those skilled in the art to which the present invention pertains
without departing from its spirit and scope. Accordingly, the scope of
the present invention is defined by the appended claims rather than the
foregoing description and the exemplary embodiments described therein.